http://bartoszmilewski.com/2014/11/04/category-the-essence-of-composition/

category.py

identity = lambda x: x

compose = lambda f, g: lambda x: g(f(x))

# Test identity function

print identity(3) is 3

# Test composition

increment = lambda x: x + 1
double = lambda x: x * 2

print compose(increment, double)(3)
print compose(double, increment)(3)

# Test composition respects identity

five = lambda x: 5

print five("WAT") is compose(identity, five)("WAT")
print five("WAT") is compose(five, identity)("WAT")